1. Field of the Invention
The present invention generally concerns three-dimensional optical memory apparatus and memory media, and methods of using such apparatus and media. The present invention particularly concerns (i) three-dimensional memories using at least two intersecting beams of radiation, (ii) the manner of using the intersecting beams and the physical and/or chemical effects of such use, and (iii) the construction of binary-stated informational memory stores, three-dimensional patterns, and/or three dimensional displays based on these effects.
2. Backcround of the Invention
The need for computerized data storage and processing has been increasing, in the past decade, at a high rate. In response to this need, semiconductor-based computer technology and architecture have greatly improved. However, increasing size and price may now be limiting still more widespread usage of high performance computers.
The major determinant of the size and price of high performance computers is the memory. The data storage requirements of new high performance computers are very great, typically many gigabytes (10.sup.12 bits) New and improved, compact, low cost, very high capacity memory devices are needed. These memory devices should be able to store many many gigabytes of information, and should randomly retrieve such information at the very fast random access speeds demanded by parallel computing.
An optical memory offers the possibility of packing binary-stated information into a storage medium at very high density, each binary bit occupying a space only about one wavelength in diameter. When practical limitations are taken into account this leads to a of about total capacity of about 10.sup.11 bits for a reasonably-sized two-dimensional optical storage medium--the amount of information contained in about 3000 normal size books. A comparison of the optical memory to existing types of computer memories is contained in the following Table 1.
TABLE 1 ______________________________________ MEMORY ACCESS TYPE CAPACITY TIME COST ______________________________________ TAPE 10.sup.10 bits 100 sec 10.sup.-5 .cent./bit DISK 10.sup.8 bits 300 msec 5 .times. 10.sup.-2 .cent./bit DRUM 10.sup.7 -10.sup.8 bits 10 msec 10.sup.-2 .cent./bit CORE 10.sup.6 bits 1 .mu.sec 2.cent./bit SEMI- 10.sup.5 bits 100 nsec 20.cent./bit CONDUCTOR OPTICAL 10.sup.9 -10.sup.12 bits 10 nsec 10.sup.-3 -10.sup.-4 .cent./bit ______________________________________
The present invention will be seen to be embodied in a 3-D optical memory which may be used in an optical memory system. Any optical memory, 3-D or otherwise, is based on light-induced changes in the optical, chemical and/or physical properties of materials.
At the present two general classes of optical recording media exist, namely phase recording media and amplitude recording media. The first are based on light-induced changes of the index of refraction (i.e., phase holograms) whereas the second refer to photo-induced changes in the absorption coefficient (i.e., hole burning).
Volume information storage is a particularly attractive concept. In a two dimensional memory the theoretical storage density (proportional to 1/.lambda..sup.2) is 1.times.10.sup.11 bits/cm.sup.2 for .lambda.=266 nm. However in a 3-D memory the theoretical storage density is 5.times.10pgy 16 pk bits/cm.sup.3. Thus the advantages of 3-D data storage versus previous two dimensional information storage media become apparent.
Volume information storage has been implemented by holographic recording in phase recording media. Reference F. S. Chen, J. T. LaMacchia and D. B. Fraser, Appl. Phys. Lett., 13, 223 (1968); T. K. Gaylord, Optical Spectra, 6, 25 (1972); and L. d'Auria, J. P. Huignard, C. Slezak and E. Spitz, Appl. Opt., 13, 808 (1974).
The present invention will be seen to implement volume writable-readable-erasable optical storage in an amplitude recording medium. One early patent dealing with three-dimensional amplitude-recording optical storage is U.S. PAT. No. 3,508,208 for an OPTICAL ORGANIC MEMORY DEVICE to Duguay and to Rentzepis, the selfsame inventor of the present invention. Duguay and Rentzepis disclose an optical memory device including a two-photon fluorescent medium which has been solidified (e.g., frozen or dispersed in a polymer). Information is written into a selected region of the medium when a pair of picosecond pulses are made to be coincident and to overlap within the selected region. The overlapping pulses create, by two-photon absorption, organic free radicals which store the information at an energy level intermediate the fluorescent level and the ground state. The radicals store the desired information for short time, until they recombine. The information may be read out by interrogating the medium with a second pair of coincident and overlapping picosecond pulses. In the case where the medium is frozen, interrogation may also be accomplished by directing a collimated infrared light beam into the selected region, thereby causing that region to liquefy and its associated radicals to undergo recombination. In each of the aforementioned cases, the interrogate beam causes the interrogated region to fluoresce. The emitted radiation is sensed by an appropriate light detector as an indication of the informational contents of the interrogated region.
This early optical memory of Duguay and Rentzepis recognizes the use of two-photon absorption only to produce excited states (e.g., singlet, doublet or triplet states) of the medium over the ground state. These excited states are metastable. For example, one preferred fluorescent medium is excitable from ground to a singlet state (by two-photon absorption in about 10.sup.-15 sec.) where it will remain about 10.sup.-8 second before fluorescing to assume a metastable triplet state. This metastable state represents information storage. Alas, it will spontaneously decay to the ground state by fluorescence after about 1 sec. (depending on temperature). The memory is thus unstable. It should be understood that the fluorescent medium of the memory is at all times the identical molecular material, and simply assumes various excited energy states.
Another previous optical system for accomplishing the volume storage of information, and other purposes, is described in the related series of U.S. Pat. Nos. 4,078,229; 4,333,165; 4,466,080; and 4,471,470 to Swainson, et al. and assigned to Formigraphic Engine Corporation. The Swainson, et al. patents are variously concerned with three-dimensional systems and media for optically producing three-dimensional elements including (i) controlled refractive index distributions, (ii) complex patterns and shapes, or (iii) physio-chemical inhomogeneities for storing data. The Swainson, et al. patents generally concern the idea that some sort of chemical reaction between two or more reactive components should be radiatively induced at selected cell sites of a 3-D memory in order to produce a stable, changed, state at these selected sites.
U.S. Pat. No. 4,471,470, in particular, describes a METHOD AND MEDIA FOR ACCESSING DATA IN THREE-DIMENSIONS. Two intersecting beams of radiation are each matched to a selected optical property or properties of the active media. In one embodiment of the method and media, called by Swainson, et al. "Class I systems", two radiation beams generate an active region in the medium through simultaneous action. Two components are typically incorporated within the medium. Both components are radiation sensitive, but to different spectral regions. The two radiation beams intersecting in a volume each produce, in parallel, an associated chemical product. The two products that are simultaneously present in the intersection region chemically react to form a desired sensible object, which may represent a binary bit of information. One or both of the radiation-induced chemical products desirably undergoes a rapid reverse reaction in order to avoid interference effects and in order to permit the three-dimensional media to be repetitively stored.
In other embodiment, called "Class II systems", one of the radiation beams must act on a component of the medium before the medium will thereafter be responsive to the other radiation beam. The class I and class II systems thusly differ by being respectively responsive to the effects of simultaneously, and sequentially, induced photoreactions.
The Swainson, et al. patents--including those patents that are not directed to information storage and that are more particularly alternatively directed to making optical elements having inhomogeneity of their refractive index, or to making physical shapes and patterns--are directed to inducing changes in a bulk media by impingent directed beams of electromagnetic radiation, typically laser light, in order that selected sites within the bulk media may undergo a chemical reaction. There are a large number of photosensitive substances that are known to undergo changes in the presence of light radiation, and the changed states of these substances are, in many cases, chemically reactive. The patents of Swainson, et al. include a great number of these photosensitive and photoreactive substances, which substances may generally also be identified from a search of the literature.
Swainson, et al. also recognize that molecular excitation from a ground state to an excited state may occur following the stepwise absorption of two photons. Swainson, et al. call this "two-photon absorption". Swainson, et al. describe that a solution of 8' allyl-6' nitro-1, 3, 3-trimethylspiro(2' N-1-benzopyran-2'-2-indoline) in benzene may be exposed to intersecting synchronized pulsed ruby laser beams transmitted through an UV elimination filter to form, at the region of intersection, a spot of color. The process of stepwise absorption of two photons in this solution, and in others, is recognized by Swainson, et al., only to produce an excited state that may form (as in the example) colored products, or which may serve as an energy transfer agent.
In making all manner of excited states--including singlet, doublet, triplet, and quartet states--the patents of Swainson, et al. describe known photochemistry. Generally chemistry, and photochemistry, that is known to work in one dimension is equally applicable in three dimensions. It is known that an electron may be knocked off an active substance so that it becomes an ion. It is known that radiation may cause a substance to dissociate a proton, again becoming an ion. It is even known how to induce spin changes and changes in parity by electromagnetic radiation. Once these changes, or others, are induced then Swainson, et al. rely on the transportive capabilities of the liquid or gaseous support media to permit a chemical reaction to transpire.
The present invention will be seen to reject this approach of inducing chemical reactions in a 3-D medium by creating one or more of the reagents with radiation. One reason why the present invention does so is because the same support medium, or matrix, that offers the transportive capabilities that are absolutely necessary to permit the chemical reactions to occur also permit, at least over time, that reagents or reaction products will migrate in three dimensions, destroying the integrity of the inhomogeneity pattern.
The three-dimensional systems of Swainson, et al., and all other such systems which the inventor is aware, fail to recognize, in combination, exactly what an active media should be, and exactly how such active media should be dealt with, in order that the active media have both (i) two stable states, which may be (ii) entered into, and subsequently interrogated, in some particular radiation-induced manner. To repeat, the existence of radiation-induced stable states in certain photoactive substances is previously known. Likewise, the manner of manipulating photoactive substances, including by the process of two-photon absorption, is also previously known. What is not known is how to put (i) the right photoactive substance undergoing the right type of transformation together with (ii) the right method for inducing such transformation and for interrogating the results thereof, in order to effectively produce usable and stable inhomogeneity patterns in a three-dimensional medium. That is the subject of the present invention.